The present application is related to the following commonly owned U.S. Patent Applications:
U.S. Patent Application entitled xe2x80x9cA BOARD-LEVEL EMI SHIELD THAT ADHERES TO AND CONFORMS WITH PRINTED CIRCUIT BOARD COMPONENT AND BOARD SURFACES,xe2x80x9d naming as inventor(s) Samuel M. Babb, Lowell E. Kolb, Brian Davis, Jonathan P. Mankin, Kristina L. Mann, Paul H. Mazurkiewicz and Marvin Wahlen, and filed concurrently herewith Ser. No. 09/812,274; and
U.S. Patent Application entitled xe2x80x9c FILLER MATERIAL AND PRETREATMENT OF PRINTED CIRCUIT BOARD COMPONENTS TO FACILITATE APPLICATION OF A CONFORMAL EMI SHIELD,xe2x80x9d naming as inventor(s) Lowell E. Kolb and filed concurrently herewith Ser. No. 09/813,257.
1. Field of the Invention
The present invention relates generally to electromagnetic interference (EMI) protective measures and, more particularly, EMI protective measures for printed circuit boards.
2. Related Art
Most countries in the world have regulations that limit the amount of electromagnetic emissions that electromagnetic equipment may produce. Electromagnetic emissions are the unwanted byproduct of high-frequency electronic signals necessary, for example, to operate an electronic microprocessor or other logic circuitry. The electromagnetic interference (EMI) that results is a problem when it interferes with licensed communications, such as television, radio, air communications and navigation, safety and emergency radios, etc. This type of interference has also been known as radio-frequency interference (RFI). See CFR 47 part 15 and ANSI publication C63.4-1992 for regulations in the United States, or CISPR publication 11 or 22 for international regulations. Also, xe2x80x9cNoise Reduction Techniques in Electronic Systemsxe2x80x9d by Henry W. Ott, serves as an excellent reference on the current art for the control of EMI, and the broader topic known as electromagnetic compatibility (EMC).
To meet EMI regulations, most electronic equipment currently employs a combination of two approaches commonly referred to as xe2x80x98source suppressionxe2x80x9d and xe2x80x9ccontainment.xe2x80x9d Source suppression attempts to design components and subsystems such that only essential signals are present in signal interconnections, and that all non-essential radio frequency (RF) energy is either not generated or attenuated before it leaves the component subsystem. Containment attempts to place a barrier around the assembled components, subsystems, interconnections, etc., so that any unwanted electromagnetic energy remains within the boundaries of the product, where it is dissipated harmlessly.
This latter approach, containment, is based on a principle first identified by Michael Faraday (1791-1867), that a perfectly conducting box that completely encloses a source of electromagnetic emissions would prevent those emissions from leaving its boundaries. This principle is employed in conventional shielded cables as well as in shielded enclosures. Conventional shielded enclosures usually consist of a metal box or cabinet that encloses the equipment. The metal box is often supplemented with additional features as necessary in an attempt to keep RF energy from escaping via the power cord and other interconnecting cables. The metal shield may be structural for example, the product enclosure itself. For example, a product enclosure might consist of a plastic structure with a conductive coating on the surface. This approach is commonly implemented in, for example, cell phones. More commonly, the metal shield is implemented as a metal xe2x80x9ccagexe2x80x9d inside the product enclosure since the EMI suppression required for the entire product or system requires that only a portion of the product be shielded. Such metallic cages are placed around components, or around subsystems when additional EMI reduction is required.
There are numerous drawbacks to the use of such metallic boxes. These drawbacks are primarily related to the lack of shielding effectiveness provided by conventional metallic boxes. For example, the metallic box creates a stagnant buffer of insulating air around the component causing the temperature of the component to increase. In such products, the enclosure typically includes cooling holes and fans to circulate air around the metallic box to dissipate the heat. In addition, electromagnetic energy often escapes the shield at gaps between the shield and the printed circuit board. Electrical gaskets and spring clips have been developed to minimize such leakage. Unfortunately, they increase the cost and complexity of the printed circuit board, and have limited success. In addition, leakage occurs because the cables and wires penetrating the shield are not properly bonded or filtered as they exit the metallic box. Further drawbacks of metallic cages include the added cost and weight to the printed circuit board assembly, as well as the limitations such metallic boxes place on the package design.
The present invention is directed to a non-electrically-conductive component cover configured to be positioned over a component mounted on a printed wiring board, the component being attached to the printed wiring board such that the component cover and printed wiring board form a component compartment dimensioned to completely encase a desired component. The apparatus and methodologies of the present invention can be used in many applications in which it is desirable to cover a printed circuit board component with a non-conductive, low profile cover that follows closely the contours of the component surface. Embodiments disclosed herein are particularly useful for use with printed circuit board coatings such as the conformal EMI shield disclosed herein. It may be required or desired to gain access to certain components mounted on a printed wiring board for troubleshooting, repair, replacement or salvage. Because coatings applied directly to the printed circuit board substantially coat the component and neighboring printed wiring board surfaces, accessing and removing a component from the printed wiring board requires that the coating be severed at those locations where the component is connected or adjacent to printed wiring board. Conventional techniques are inaccurate and can damage the printed circuit board, the component, and the electrical and physical connection between the two.
A number of aspects of the invention are summarized below, along with different embodiments that may be implemented for each of the summarized aspects. It should be understood that the embodiments are not necessarily inclusive or exclusive of each other and may be combined in any manner that is non-conflicting and otherwise possible. It should also be understood that these summarized aspects of the invention are exemplary only and are considered to be non-limiting. Also, various aspects of the present invention and embodiments thereof provide certain advantages and overcome certain drawbacks of conventional techniques. Not all aspects and embodiments share the same advantages and those that do may not share them under all circumstances. These disclosed aspects, some of which are summarized below, are not to be construed as limiting in any regard; they are provided by way of example only and in no way restrict the scope of the invention.
In one aspect of the invention, a component cover is disclosed. The component cover is non-electrically-conductive and is configured to be positioned over a component mounted on a printed wiring board. The component cover is configured to be attached to the printed wiring board such that the component cover and printed wiring board form a component compartment dimensioned to completely encase the desired component. The component cover has an exterior surface shape suitable to have a coating applied thereto. Preferably, the component compartment has an exterior surface that is contoured and without sharp edges, cavities or other abrupt changes. The resulting component compartment can be airtight and, in certain embodiments, evacuated.
In one preferred embodiment, the component cover is configured such that, when attached to the printed wiring board, the component cover has a minimal profile, not adding significantly to the dimensions of the printed circuit board and its components. In one embodiment, this is achieved with a component cover formed of walls having a minimal thickness and an interior surface that is immediately adjacent to surfaces of the component. In other words, the component cover has a contoured surface that follows generally a contour of the component.
In accordance with certain embodiments, the component cover includes a dome having a closed top, an open bottom, and walls extending between the top and bottom to form a recess suitable for receiving the component. The component cover also includes a flange surrounding the open bottom to be secured to the printed wiring board. The component cover can be unitary or integral.
Embodiments of the component cover of the present invention include different features to facilitate accessing the encased component. In one embodiment, the component cover is configured such that at least a portion of the cover can be severed and separated from the printed wiring board to provide the desired access. In another embodiment, the component cover is formed of a dome connected to a detachable flange. In a further embodiment, the component cover has a removable top, lid or has an aperture with a removable cover or insert. Alternatively, the component cover includes a line of severability that traverses the component cover so as to define a portion of the component cover sufficiently large to provide access. The line of severability can be, for example, a line of weakening such as a crease or fold line that facilitates the severing of a portion of the component cover defined by the line of severability. In still other embodiments, the component cover is sufficiently thin and formed from a material that can be cut manually. In a further embodiment, in combination with the line of severability, a pressure-rupturable dome is implemented so that, in response to a manual force applied radially inward, the dome ruptures and severs along the line of severability.
The component cover dome can have any shape suitable for the component to be covered. For example, the dome can be configured with a surface of revolution about a vertical axis such as a half-sphere, or it can have an arbitrary shape. Similarly, the flange can have any suitable configuration, typically with a planar bottom surface to mate with the printed wiring board.
In another aspect of the invention, a printed circuit board is disclosed. The printed circuit board includes a printed wiring board with one or more components mounted thereon. The printed circuit board also includes one or more of the non-electrically-conductive component covers summarized above.
In a still further aspect of the invention, a method for providing a printed circuit board with one or more components encased in a component compartment suitable to be coated with an atomized coating is disclosed. The method includes (a) determining the dimensions of the component cover dome; (b) determining dimensions of the component cover flange; (c) manufacturing the component cover based on the dome and flange dimensions; and (d) attaching the component cover to the printed wiring board to form the component compartment. Preferably, the last step includes evacuating and sealing the component compartment.
Further features and advantages of the present invention as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings. In the drawings, like reference numerals indicate identical or functionally similar elements. Additionally, the left most one or two digits of a reference numeral identify the drawing in which the reference numeral first appears.